This invention relates to programmable logic device integrated circuits (“PLDs”), and more particularly to circuitry on PLDs that is variable to facilitate connecting the PLD to various external signal sources.
PLDs are typically intended to be relatively general-purpose devices. The market for a PLD product tends to be larger if the number of different applications in which that product can be successfully employed is greater. A PLD typically interacts with other devices in a system. The wider the range of different signals the PLD can successfully accept from such other system components, the greater the range of possible uses of the PLD, and therefore the greater the potential market for the PLD. By “different signals” in the preceding sentence, it is meant that these different signals have different characteristics or specifications. Examples of such different characteristics are (1) a difference in ground voltage level between the signal source and the PLD, or (2) a difference in common mode voltage level between the signal source and the PLD. Such signals may be data signals (e.g., high-speed serial data signals) or clock signals. High-performance analog receiver or clock buffers on a PLD tend to be optimized to work for a small range of common mode voltages. This range may be limited due to concerns about oxide overstress or keeping transistors in the proper operating range. Again, because a PLD should preferably meet the needs of a wide-range customer base, it would be desirable to be able to either move the commode mode of the incoming signal at a point prior to the PLD's analog receiver or clock buffer, or to limit that incoming signal's output swing to not overstress the receiver or buffer. Among other possible features, the present invention may allow the PLD to change the corner frequency of input signal interface circuitry, for example, in a programmable manner (e.g., using programmable configuration random access memory (“CRAM”) bits on the PLD), or in a dynamic manner. This can help the PLD support different applications where the frequency of interest (i.e., of a received signal) is different.